Serials: Don't force a Receive Unit Template in Alt Serials Control

For Evergreen 2.2+:

    Mary Llewellyn reported that she used to be able to use serials and
    perform copy-less receiving without any value in this field.

    The requirement that a value be set in the Receive Unit Template field
    of serial distributions was added (by me) in 2.1 to the alternate serials
    control interface, but is not enforced at lower levels.

    I now believe that requiring a value here must have been a ham-handed
    way to avoid a difficult-to-troubleshoot issue in copy-FUL receiving,
    but copy-LESS serials receiving workflows indeed don't need this, so
    this commit removes the requirement.

In other words, I think this is a bugfix and is suitable for backport through 2.2 (or 2.1? pending clarification on what the oldest community-supported release is at the moment).

http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/senator/serial-dont-require-rut